1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and a semiconductor device and an electronic device which are manufactured using the method.
Note that in this specification, a semiconductor device means a device which can function by utilizing semiconductor characteristics, and a display device or the like as well as a semiconductor circuit is included in the semiconductor device.
2. Description of the Related Art
Integrated circuits using an SOI (silicon on insulator) substrate, instead of using a bulk silicon wafer, have been developed. By utilizing the characteristics of a thin single crystal silicon layer formed over an insulating layer, transistors formed in the integrated circuit can be separated from each other completely. Further, since the fully depleted transistors can be formed, a semiconductor integrated circuit with high added value such as high integration, high speed driving, and low power consumption can be realized.
As a manufacturing method of an SOI substrate, a hydrogen ion implantation separation method in which hydrogen ion implantation and separation by the hydrogen ion implantation are combined is known. A typical process of the hydrogen ion implantation separation method is described below.
First, hydrogen ions are implanted into a silicon wafer to form a damaged region at a predetermined depth from the surface. Next, a silicon oxide film is formed by oxidizing another silicon wafer which serves as a base substrate. After that, the silicon wafer into which the hydrogen ions are implanted is bonded to the silicon oxide film of the other silicon wafer, so that the two silicon wafers are attached to each other. Then, heat treatment is performed thereon, whereby the wafers are cleaved from each other with the damaged region used as a cleavage plane. Note that different heat treatment from the heat treatment at the time of cleavage is performed in order to improve the bonding force in the attachment.
In addition, a method for forming a single crystal silicon layer over a glass substrate by the hydrogen ion implantation separation method is known (for example, see Patent Document 1: Japanese Published Patent Application No. H11-097379). In Patent Document 1, a separation surface is mechanically polished in order to remove a defect layer which is formed by ion implantation and steps of several nm to several tens of nm at the separation surface.
A glass substrate is an inexpensive substrate with larger area than a silicon wafer, and is mainly used when a liquid crystal display device is manufactured. By using a glass substrate as a base substrate, an inexpensive and large-area SOI substrate can be manufactured.
However, the glass substrate has a predetermined strain point and low heat resistance. Therefore, the glass substrate cannot be heated at a temperature which exceeds a heat resistance temperature thereof, and the process temperature is limited to be less than or equal to the strain point. That is, there is also a process temperature limit when crystal defects are reduced and surface unevenness is reduced in a separation surface. In addition, there is a process temperature limit also in manufacturing a transistor using a single crystal silicon layer attached to a glass substrate.
Further, in a case of a large-sized substrate, an apparatus and a processing method which can be used are also limited. For example, the use of the mechanical polishing of the separation surface described in Patent Document 1 is not practical for a large-area substrate, in terms of processing accuracy, cost for an apparatus, or the like. However, to bring out the characteristics of a semiconductor element, it is necessary that the surface unevenness in the separation surface and the defect density in a semiconductor layer be suppressed to a certain value or less. Particularly when the single crystal silicon layer is used as an active layer of a semiconductor element (e.g., a channel formation region of a transistor), this point is extremely important.
As described above, in the case where a substrate which is large in area and low in heat resistance, such as a glass substrate, is used as a base substrate, it is difficult to suppress surface unevenness and the defect density of a semiconductor layer and to obtain desired characteristics.